The present invention relates to a technique for correcting recording density irregularity in image recording by using a line head.
To prepare a printing plate from a color sheet of manuscript to thereby perform printing on a large number of sheets of paper, proof printing is performed and then final printing is performed after the finished state of the proofs is checked. In the proof printing process, a color proof is produced and the image quality of the color proof is checked. The color proof is obtained by the steps of: thermally transferring an image to a receiver sheet by a thermal printer; and transferring the image formed on the receiver sheet to a sheet of paper.
As is known well, in an image recording apparatus such as a thermal printer, or the like, while a glaze formed by arrangement of heating resistors corresponding to one-line's pixels in one direction (a first main scanning direction) is relatively moved, together with a printing medium, in the subsidiary scanning direction substantially perpendicular to the direction of the arrangement of the heating resistors in the condition that the glaze is slightly pressed against the printing medium, the respective heating resistors in the glaze are heated in accordance with image data of a recording image. As a result, an image is thermally transferred to the printing medium to thereby form a recording image.
In the image recording apparatus using the aforementioned recording method, when, for example, a recording image is formed by use of image data having one and the same predetermined-gradation value, recording density irregularity such as shading, or the like, in which the density of the formed recording image varies in accordance with the heating resistors, may occur. This is inevitable because the shape of the glaze of the line head is not always uniform. In some image processing apparatus, density irregularity correction is performed for image data in advance to prevent lowering of image quality caused by density irregularity.
A specific example of the density irregularity correction method is the following method.
First, an image is recorded on the basis of image data having a uniform gradation in the first main scanning direction of the line head. The thus recorded image is scanned by a densitometer such as a scanner, or the like, moving relatively in the subsidiary scanning direction of the line head to thereby detect printing density. The printing density obtained for each pixel is compared with the gradation value of the image data to thereby correct the gradation value of the image data.
In the conventional density irregularity correction method described above, however, printing densities in different positions of the recording image are detected individually by detection elements of the scanner. Accordingly, the printing-density value may vary in accordance with the individual sensitivity characteristic of the detection elements of the scanner. Generally, the sensitivity characteristic of the detection elements of the scanner is not uniform in the strict sense, so that printing densities to be detected as the same value originally are detected as different density values. As a result, the correction values which are set on the basis of the detected printing-density values become incorrect. There arises a problem that appropriate density irregularity correction cannot be made.